linux-kernel - Re: [PATCH V3 1/5] genirq/affinity: don't mark 'affd' as const

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Date:   Wed, 13 Feb 2019 15:37:11 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Sagi Grimberg <sagi@...mberg.me>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V3 1/5] genirq/affinity: don't mark 'affd' as const

On Wed, Feb 13, 2019 at 10:41:55PM +0100, Thomas Gleixner wrote:
> Btw, while I have your attention. There popped up an issue recently related
> to that affinity logic.
> 
> The current implementation fails when:
> 
>         /*
>          * If there aren't any vectors left after applying the pre/post
>          * vectors don't bother with assigning affinity.
> 	 */
> 	if (nvecs == affd->pre_vectors + affd->post_vectors)
>     		return NULL;
> 
> Now the discussion arised, that in that case the affinity sets are not
> allocated and filled in for the pre/post vectors, but somehow the
> underlying device still works and later on triggers the warning in the
> blk-mq code because the MSI entries do not have affinity information
> attached.
>
> Sure, we could make that work, but there are several issues:
> 
>     1) irq_create_affinity_masks() has another reason to return NULL:
>        memory allocation fails.
> 
>     2) Does it make sense at all.
> 
> Right now the PCI allocator ignores the NULL return and proceeds without
> setting any affinities. As a consequence nothing is managed and everything
> happens to work.
> 
> But that happens to work is more by chance than by design and the warning
> is bogus if this is an expected mode of operation.
> 
> We should address these points in some way.

Ah, yes, that's a mistake in the nvme driver. It is assuming IO queues are
always on managed interrupts, but that's not true if when only 1 vector
could be allocated. This should be an appropriate fix to the warning:

---
diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 022ea1ee63f8..f2ccebe1c926 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -506,7 +506,7 @@ static int nvme_pci_map_queues(struct blk_mq_tag_set *set)
 		 * affinity), so use the regular blk-mq cpu mapping
 		 */
 		map->queue_offset = qoff;
-		if (i != HCTX_TYPE_POLL)
+		if (i != HCTX_TYPE_POLL && dev->num_vecs > 1)
 			blk_mq_pci_map_queues(map, to_pci_dev(dev->dev), offset);
 		else
 			blk_mq_map_queues(map);
--